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【導(dǎo)讀】一些短語諸如“超級(jí)公路信息”已隨處可見，信息處理成為一個(gè)迅。速增長為每年幾十億美元的產(chǎn)業(yè)。得到廣泛的認(rèn)可。這種議論推動(dòng)了對(duì)強(qiáng)大及靈活的數(shù)據(jù)庫管理系統(tǒng)的不斷增長的。數(shù)據(jù)庫是一組數(shù)據(jù)的集合，它可典型地描繪一個(gè)或多個(gè)相關(guān)組織的活動(dòng)。來說，在文件中存儲(chǔ)數(shù)據(jù)須書寫特定的應(yīng)用代碼來管理它。一般來說，數(shù)據(jù)庫管理系統(tǒng)領(lǐng)域是計(jì)算機(jī)科學(xué)的一個(gè)縮影。取，數(shù)據(jù)庫管理系統(tǒng)將繼續(xù)增加其重要性。在商業(yè)方面，數(shù)據(jù)庫管理系統(tǒng)代表了。早期的系統(tǒng)是順序組。通過索引隨機(jī)訪問數(shù)據(jù)成為可能。網(wǎng)絡(luò)型數(shù)據(jù)庫的速度及多樣性使其在企業(yè)中得到廣泛。當(dāng)文件或記錄間的關(guān)系不能用鏈表達(dá)時(shí)，使用關(guān)系型數(shù)據(jù)庫?！瓣P(guān)系”，就是一個(gè)簡單的非結(jié)構(gòu)列表。一個(gè)關(guān)系數(shù)據(jù)庫能夠規(guī)定列中的一個(gè)單元格是否為空。
【正文】 schoolclass enrollment the STUDENT table has a manytomany relationshipwith the CLASS table—one student may take one or more classes and agiven class may have one or more students. The intersection tableSTUDENT_CLASS would contain the binations of STUDENT andCLASS to track which students are in which Introduction The term database is often to describe a collect: on of related filesthat is anized into an structure that provides differentpeople variances to the same data. For example the department motorvehicles has a data base of all the informal： on drivers licenses in allcounties and cities in state. Some of the information in the files criminal record convictions， and the like may be made available topolice departments other information . a balance due on a speedingticket maybe made available to licensing bureaus and still otherinformation . number and type of convictionsmay go to insurancepanies． A Data Base Management System DBMS is an extremelyplicated set of software programs that controls the anizationstorage and retrieval of datafelids records and filesin a Database. It alsocontrols the security and integrity of the databases． The DBMS acceptsrequests for data from the application program and instructs the operatingsystem to transfer the appropriate data. Tasks that would be timeconsuming to acplish manually aremore practical the aid of the puter. In principle a database in aputer is not different from a database recorded on paper and filed incabis． But the puter does the tedious work of maintaining andaccessing a database and does it fast. A puterized database that cando all this is known as a database management system or DBMS 13 forshort. There are a number of ways to store information in a puter butnot all of these are true database management systems. When a DBMS is used information systems can be changed muchmore easily as the anization’s information requirements change. Newcategories of data can be added to the database without disruption to theexisting Management System DBMS is able to access and retrieve data from nonkey record is the DBMS is able to structure and tie together the logicallyrelated data from several large . Logical Structures Identifying these logical relationships is a job of the dataadministrator. A data definition language is used for this purpose. TheDBMS may then employ one of the following logical structuringtechniques during storage access and retrieval operations: structures. In this logical approach records are linked togetherby the use of pointers. A pointer is a data item in one record that identifiesthe storage location of another logically related record. Records in acustomer master file for example will contain the name and address ofeach customer and each record in this file is identified by an accountnumber. During an accounting period a customer may buy a number ofitems on different days. Thus the pany may maintain an invoice fileto reflect these transactions. A list structure could be used in this situation to show the unpaidinvoices at any given time. Each record in the customer file wouldcontain a field that would point to the record location of the first invoicefor that customer in the invoice file. This invoice record in turn wouldbe linked to later invoices for the customer. The last invoice in the chainwould be identified by the use of a special character as a pointer. tree structures. In this logical approach data unitsare structured in multiple levels that graphically resemble an quotupsidedownquot tree with the root at the top and the branches formed a superiorsubordinate relationship in a hierarchical treestructure. Below the singleroot data ponent are subordinate elementsor nodes each of which in turn quotownquot one or more other elements ornone. Each element or branch in this structure below the root has only asingle owner. Thus a customer owns an invoice and the invoice hassubordinate items. The branches in a tree structure are not connected. Structures. Unlike the tree approach which does notpermit the connection of branches the work structure permits theconnection of the nodes in a multidirectional manner. Thus each nodemay have several owners and may in turn own any number of other dataunits. Data management software permits the extraction of the needed.
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